package yuxin;

import java.util.Arrays;
import java.util.Scanner;

public class T3 {

    //    public static void main(String[] args) {
//        double result = 0.0;
//        System.out.println(
//                ((5/6.0) * (5/7.0) * (5/8.0) * (5/9.0)) +
//                        ((1/3.0) * (1/4.0) * (1/5.0) * (1/6.0)) +
//                        ((2/3.0) * (2/5.0) * (2/6.0) * (2/7.0)) +
//                        ((3/4.0) * (3/5.0) * (3/7.0) * (3/8.0)) +
//                        ((4/5.0) * (4/6.0) * (4/7.0) * (4/9.0))
//        );
//    }
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int k = sc.nextInt();
        int[] a = new int[n];
        for(int i=0 ; i<n; i++) {
            a[i] = sc.nextInt();
        }
        Arrays.sort(a);

        double[] winChange = new double[n];
        for(int i=0 ; i<n; i++) {
            double res = 1.0;
            for (int j=0; j<n; j++) {
                if (j == i) continue;
                res *= a[i]/(double)(a[i]+a[j]);
            }
            winChange[i] = res;
        }
        if (k==0) {
            double res = 1.0;
            for(int i=0; i<n; i++) {
                res *= (1.0 - winChange[i]);
            }
            System.out.printf("%.2f", res);
            return;
        } else if (k==1) {
            double res = 0.0;
            for (int i=0; i<n; i++) {
                res += winChange[i];
            }
            System.out.printf("%.2f", res);
            return;
        } else if(k==2) {
            double res = 0.0;
            for (int i=0; i<n; i++) {
                double temp = 0.0;
                for(int j=0; j<n; j++) {
                    if (i==j)   continue;
                    temp += winChange[i] * winChange[j];
                }
                res += temp;
            }
            System.out.printf("%.2f", res);
        }else if (k==n) {
            double res = 1.0;
            for (int i=0; i<n; i++) {
                res *= winChange[i];
            }
            System.out.printf("%.2f", res);
            return;
        }
    }
}
